1. Field of the Invention
The present invention relates to a method and mechanism for firing gas turbines with liquefied natural gas for the driving of generators.
2. Description of the Prior Art
One method of thermal power generation is by gas turbines which are run by the burnt gas of combustion, and it is usually applied to power plants having a capacity up to several tens of thousand of kilowatt. There is a new method of thermal power generation which is commonly called combined power generation. This new method is based on a gas turbine and a steam turbine in combination. It offers an advantage of being started and stopped easily and permitting easy control for load variation. Moreover, it is more efficient than the ordinary steam power generation. Thus it is considered to be promising. The general trend in combined power generation is toward firing gas turbine with liquefied natural gas which is a clean fuel. The combined power generation resorting to liquefied natural gas will become more important in the countries and areas where energy demand is expected to increase in the future. Liquefied natural gas is desirable from the standpoint of stable supply in view of its huge reserves and also from the standpoint of environmental protection.
In the meantime, a gas turbine has the disadvantage of decreasing in output with the increasing atmospheric temperature. This is because an increase in atmospheric temperature decreases the density of combustion air being supplied to the gas turbine. The lower the density of air, the smaller the mass of air in the same volume. Unfortunately, the atmospheric temperature is highest in the afternoon in summer when the electric power consumption is highest for air conditioners. In other words, the output of gas turbines is lowest when the electric power consumption is highest. This prevents the effective use of combined power generation.
A common practice to overcome this disadvantage is to use an air precooling system (as shown in FIG. 17) for gas turbines fired with natural gas. Liquefied natural gas (LNG) indicated by la is introduced into an LNG vaporizer 1' before it is supplied to a turbine 51. The LNG vaporizer 1' is provided therein with a heat exchanger tube 12. The inlet of the heat exchanger tube 12 is connected to a passage L1 through which LNG 1a is introduced from outside. The outlet of the heat exchanger tube 12 is connected to a passage L2 through which natural gas (NG) is supplied to the turbine 52. The rotation of the turbine 52 is transmitted to the generator 6.
The LNG vaporizer 1' is supplied with a heat transfer medium 1b through a passage L3. The heat transfer medium 1b heats and vaporizes LNG 1a passing through the heat exchanger tube 12. After passing through the LNG vaporizer 1' the heat transfer medium 1b is cooled by heat exchange through the heat exchanger tube 12 and discharged. The heat transfer medium 1b is recycled through passages L4 and L3 by a recycling pump 3 installed in the passage L4.
Between the passages L4 and L3 is an air precooler 4, so that heat exchange takes place between the cooled heat transfer medium and the air 1c introduced from outside. The cooled air resulting from heat exchange is introduced into an air compressor 51 of the turbine unit 5. On the other hand, the heat transfer medium 1b which has been heated as the result of heat exchange with air 1c is introduced again into the LNG vaporizer 1' for the heating of LNG 1a.
The above-mentioned system performs heat exchange between LNG 1a and air 1c by means of the heat transfer medium 1b, and the heat exchange heats and vaporizes LNG 1a and precools air 1c, increasing its density. This leads to a substantial increase in the amount of combustion air to be supplied to the turbine 52 and hence an increase in turbine output.
The general method of operating a gas turbine fired with liquefied natural gas has been explained above with reference to FIG. 17. The method and apparatus for increasing the output of the turbine 52 by cooling air 1c by heat exchange between LNG 1a and air 1c are disclosed in Japanese Patent Publication No. 2771/1984, Japanese Patent Laid-open Nos. 47625/1981 and 142219/1989, and Japanese Utility Model Laid-open Nos. 174730/1982, 41537/1985, and 186908/1988.
The prior art technologies, however, generally lack the technical idea of storing "cold" although they involve heat exchange for air cooling, except for Japanese Patent Laid-open No. 142219/1989. The method disclosed in it employs a mixture of water and ethylene glycol (antifreezing fluid) as the heat transfer medium. Storing "cold" by means of the sensible heat of the heat transfer medium alone needs a considerably large storage tank for the heat transfer medium.
The disadvantage of operating a gas turbine by firing with liquefied natural gas in the conventional manner as shown in FIG. 17 is that the cooling of air 1c by the cooling medium 1b is by far less than expected, because the amount of LNG 1a supplied to the heat exchanger tube 12 is determined by the turbine 52 and the ability of LNG 1a to cool the heat transfer medium 1b is limited. As the result, the amount (in weight) of air 1c supplied to the turbine unit 5 does not increase so much as expected, nor does the output of the turbine 52. The precooling of air 1c by heat exchange with LNG 1a in the conventional manner (as disclosed above) is not enough to operate the gas turbine to make up for power shortage when power consumption is at its peak.
Incidentally, Japanese Patent Laid-open No. 142219/1989 shown above discloses a system which has a heat accumulator (like a pool) installed in the recycling passage for the heat transfer medium, so that the heat transfer medium that has undergone heat exchange with LNG is collected in the heat accumulator and the freezing latent heat is accumulated therein. The thus accumulated "cold" can be used to cool air when the turbine load is high. Therefore, this system is superior to those in other disclosures. This system, however, has the disadvantage of using as the heat transfer medium a mixture of water and ethylene glycol (antifreezing fluid) and resorting to "cold" accumulation by means of the sensible heat of the heat transfer medium (which is apparent from the use of antifreezing fluid). Therefore, a considerably large storing tank is required to store "cold" by cooling the heat transfer medium stored in the heat accumulator like a pool.
In the meantime, Japanese Patent Laid-open No. 142219/1989 mentions that the storing of "cold" in the form of ice is effective. However, it is impossible that the heat transfer medium freezes in the heat accumulator, because the heat accumulator is installed outside the LNG vaporizer to cool the heat transfer medium and the cooled heat transfer medium is introduced into the heat accumulator through the piping. This means that the heat transfer medium is transferred in the unfrozen state and the heat transfer medium increases in temperature during transportation due to heat supplied from the atmosphere. It is concluded that storing "cold" in the form of ice is unrealistic.
The present invention was completed to address the above-mentioned problems. Accordingly, it is an object of the present invention to provide a method for effectively operating a gas turbine by firing with liquefied natural gas. The object is achieved by effectively storing "cold" for the heat transfer medium to cool air.